1. Field of the Invention
The invention relates generally to the field of seismic exploration of the Earth's subsurface. More particularly, the invention relates to methods for processing seismic signals for the purpose of identifying and/or locating fractures in the Earth's subsurface.
2. Background Art
Seismic exploration of the Earth's subsurface is used for, among other purposes, determining the structure, mineral composition and/or fluid content of subsurface formations. Generally, seismic exploration methods known in the art include deploying one or more seismic energy sources at selected positions on or near the Earth's surface, and deploying an array of seismic sensors on or near the Earth's surface at known positions proximate to the seismic energy source(s). When seismic exploration is performed on land, the seismic energy sources are typically explosives or vibrators, and the seismic sensors are typically geophones disposed at spaced apart locations along a sensor cable. When seismic exploration performed in a body of water, the source(s) may be an air gun or array of air guns, water guns or explosives, and the sensors are typically hydrophones disposed along a “streamer” cable towed in the water near the water surface. For both land and water seismic exploration, Periodically, the sources are actuated, and signals detected by the sensors in the array are recorded. The recorded signals are indexed with respect to the time of actuation of the source.
Seismic energy emanates outwardly from the seismic energy source, and travels through the subsurface formations until it reaches a subsurface acoustic impedance boundary. At the boundary, some of the seismic energy is reflected back toward the surface where it is detected by the sensors. Structure and composition of the subsurface Earth formations is inferred by various characteristics of the detected seismic energy, including seismic energy travel time from the source to each detector, and the phase, amplitude and frequency content of the detected seismic energy, among other characteristics.
An important assumption concerning the propagation of seismic energy through the Earth from the source to the detectors is that the Earth consists of substantially horizontally layered, laterally continuous media that have lateral extent greater than the lineal distance between the source and the most distant one of the seismic detectors. It is also assumed that a substantial portion of the detected seismic energy results from so-called “specular” reflections of the seismic energy at one or more acoustic impedance boundaries in the subsurface. A specular reflection is defined as “a sharply defined beam resulting from reflection off a smooth surface.”
Such assumptions about the subsurface structure of the Earth result in methods for generating images of the Earth's subsurface structure that are limited to one-half the lineal distance between the source and the most distant detector. Imaging other parts of the Earth's subsurface is thus typically performed by moving the seismic source and the detectors and repeating the signal acquisition techniques described above.
It is known in the art that some types of subsurface Earth structures do not fit the foregoing assumptions of lateral continuity and horizontal layering. In particular, so-called “fractured” formations typically are not well represented by a model based on laterally continuous, horizontally layered media. Fractured formations may have very limited or irregularly distributed lateral extent, they may be oriented other than horizontally, and they may have unknown geographic compass direction (azimuth) with respect to their lateral extent.
Determining the existence of and areal extent of fractured formations is of continuing interest in subsurface Earth exploration because valuable resources, such as petroleum, are frequently associated with fractured formations. Methods are known in the art for evaluating fractured Earth formations. See, for example, Kanasewich, E. R., et al., Imaging discontinuities on seismic sections, Geophysics, vol. 53, no. 3, pp. 334–345, Society of Exploration Geophysicists (1988); Landa, E. et al., Seismic monitoring of diffraction images for detection of local heterogeneities, Geophysics, vol. 63, no. 3, pp. 1093–1100, Society of Exploration Geophysicists (1998); and Khaidukov, V. et al., Diffraction imaging by a focusing-defocusing approach, transactions of the 73rd annual meeting, Society of Exploration Geophysicists (2001).
More recently, a number of publications have been made concerning methods for evaluating subsurface fractures. For example, at a geophysical science conference held in 2003, more than twenty papers were presented on the subject of fractures. See, Transactions of the 75th annual meeting, Society of Exploration Geophysicists (2003). A number of the papers from the above conference describe methods adapted from seismic signal processing “edge detection” techniques. Other papers from the foregoing conference describe azimuthal amplitude versus offset (AVO) techniques for fracture orientation determination. However, azimuthal AVO data are not available for all areas. Furthermore, azimuthal AVO has not proven to be particularly effective in marine seismic data recording and analysis, unless some specially designed recording methods and systems are used, for example, four component (4C) ocean bottom cables (OBCs). Examples of using ocean bottom cables are described in several references. One such technique is described in U.S. Pat. No. 4,486,865 issued to Ruehle. Pairs of detectors each comprise a geophone and a hydrophone. A filter is applied to the output of at least one of the geophone or hydrophone in each pair so that the frequency content of the filtered signal is adjusted. The adjustment to the frequency content is such that when the filtered signal is combined with the signal from the other sensor, the ghost reflections cancel. U.S. Pat. No. 5,621,700 issued to Moldovenu also discloses using at least one pair of sensors in a method for attenuating ghosts and water layer reverberations. U.S. Pat. No. 4,935,903 issued to Sanders et al. discloses a method for reducing the effects of water later reverberations which includes measuring pressure at vertically spaced apart depths, or by measuring pressure and particle motion using sensor pairs. The method includes enhancing primary reflection data for use in pre-stack processing by adding ghost data. U.S. Pat. No. 4,979,150 discloses a method for marine seismic exploration in which output of substantially collocated hydrophones and geophones are subjected to a scale factor. The collocated hydrophones and geophones can be positioned at the sea floor or above the sea floor.
However, OBC data are not available in all areas, and acquisition and processing of OBC data continues to be relatively difficult and expensive. Thus there continues to be a need for effective imaging of subsurface diffractors, including fractured zones, which does not require specialized data recording techniques or equipment, and may be applicable to seismic data recorded using older acquisition techniques wherein AVO analysis may not be possible.